What is Keum Boo? The Ancient Art of Adding Gold to Silver
What is Keum Boo?
Keum Boo is an ancient Korean technique where 24 carat gold foil is fused onto fine silver using heat and pressure. The word Keum Boo literally translates as "attached gold".
How does it work? The science bit!
Sterling silver is made up of 92.5% pure silver and 7.5% copper. When sterling silver is heated the copper oxidizes easily and forms a microscopic barrier on the surface of the silver which prevents the gold from bonding to the silver. Therefore, the surface of the sterling silver has to be prepared before the 24 kt gold is attached.
To do this the sterling silver is repeatedly heated, cooled and pickled. This will dissolve the copper oxide on the surface and leave behind a layer of fine silver. This is called depletion gilding. I like to take it a little bit further with the process of reticulation where I heat the silver until the surface becomes rippled and I can carefully heat and move the metal surface using a jewellers torch. Once it’s cooled down, a bath in jewellers pickle will remove the surface oxides and the process is repeated many times to build up the fine silver at the surface. It can take a while but it’s great fun and always makes me think of the surface of the moon.
Once the silver has been turned into the piece of jewellery, the final stage is to apply the 24-carat gold foil. The piece is heated on a hot plate, this allows the silver to reach a high temperature without any danger of melting. A piece of gold foil is then applied to the surface and burnished on. The gold bonds to the silver through heat, pressure and atomic diffusion.
Although the gold is permanently bonded to the surface the thin gold surface can still wear with repeated friction, especially on rings.
Why don’t I use fine / pure silver?
It does seem more logical to simply use pure / fine silver to start with but fine silver is 99.9 % silver which is much softer than sterling silver (92.5 %). I prefer using sterling silver for the strength it provides and tend to use fine silver for bezel walls.
Why I love using Keum Boo
On a cold Winter’s day, sitting at a hot plate applying little shapes of 24 carat gold foil to silver must be one of my favourite ways to spend time. Sitting burnishing on the gold always makes me think about how truly handmade the piece is. However, it’s not as pleasant on a hot summer’s day!
I love being able to add tiny details of gold to a piece of silver and adding a patina to the silver really enhances the gold and contrasts beautifully. With the cost of gold being so high at the moment it’s also a great way to work with gold.
Keum Boo pieces by Thistledown Wishes
It's such a lovely way to add a touch of gold to a piece of silver jewellery. These are some of my current pieces, you can find them and others through this link and I'm sure that I will be adding more in the future.
Top tips
Use a shaped hole punch to create little shapes in the gold foil.
Make sure that the piece is very clean, oil from your fingers can stop the gold attaching to the surface. Use tweezers to lift the foil.
If the gold is struggling to attach but you know that the surface is clean then check the temperature of the hot plate is high enough.
If you find the heat uncomfortable to work with then you could apply the gold to the surface of the silver before you switch on the hot plate. This helps you to be able to arrange the pieces as you want them without having to hold your hands over the hot plate for too long.
Wearing heat resistant gloves really helps to protect your hands from the heat of the hot plate, I use BBQ gloves.
Applying a patina to darken the silver creates a beautiful contrast but do this as the very last stage.
Don't polish the surface with a rotary hand piece as this can damage the gold but putting your final piece in the tumbler is safe (assuming there are no stones set etc).
I hope you've enjoyed reading this and I would love to see any of your keum boo pieces.
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